1. Field of the Invention
This invention relates generally to the field of semiconductor circuit design, and more particularly to the design of improved current source circuits.
2. Description of the Related Art
A current source is an essential circuit component of many analog integrated circuits. To put simply, a current source is a circuit that delivers or absorbs current. In theory, an ideal (independent) current source should deliver a substantially constant current, unaffected by surrounding environmental factors and/or any other variables in the circuit. For example, current sources should preferably not be influenced by variations of the load, supply voltage, or changes in temperature, to ensure stable and predictable operation of the system and/or circuit relying on the current sources. Circuit components that may be sensitive to temperature variations, such as transistors, should especially be supplied with temperature-independent or controllably temperature-dependent currents for reliably predictable operation.
Since most electrical components have a temperature coefficient, current sources comprising electrical components are typically affected by temperature variations. When an electrical component, e.g. a resistor has a Positive Temperature Coefficient (PTC), that resistor experiences an increase in electrical resistance as its temperature increases. The higher the coefficient, the greater the increase in electrical resistance for a given increase in temperature. In contrast, when a resistor has a negative temperature coefficient (NTC), its conductivity rises with increasing temperature, typically within a defined temperature range.
Taking into account the temperature coefficients and overall electrical characteristics of the various components from which a current source may be formed, current sources can be designed to output currents that have a positive temperature coefficient (PTC) or a negative temperature coefficient (NTC). In general, depending on the given circuit configuration and/or topology, a current may be a PTC current or an NTC current, among others. A PTC current will increase as temperature increases, and decrease as temperature decreases, while an NTC current will decrease as temperature increases, and increase as temperature decreases.
In analog integrated circuits, current sources are often used in place of resistors to generate a current without introducing attenuation in the signal path where the current source is coupled. For example, in CMOS circuits, the drain of a field effect transistor (MOSFET) can behave as a current source when properly connected to an external source of energy (such as a supply voltage) due to the intrinsically high output impedance of the MOSFET when used in a current source configuration. Although such current sources are ideally expected to behave in a stable manner, their operation can be noticeably affected by variations in environmental factors such as temperature and supply voltage.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.